46 FRANK BLAIR HANSON 



Parker ('91), accepting in common with others the validity of 

 Ruge's theory of a costal sternum in reptiles, birds, and mammals, 

 but being unable to relate a sternum of such derivation with the 

 sterna of the Ichthyopsida, suggested that there must be two 

 distinct types of sterna: 1) a costal sternum, characteristic of the 

 Amniota, and 2) a coracoidal or clavicular sternum, character- 

 istic of the Ichthyopsida. 



This classification of the sternum was adopted by Howes 

 ('91) who says, "the distinction indicated by the two terms 

 'costal sternum' and 'coracoidal sternum' is but the expression of 

 a fundamental morphological difference between the two struc- 

 tures." Howes slightly altered the terminology of Parker. He 

 would distinguish between a 'coracoidal archisternum'of the Ich- 

 thyopsida and a 'haemocoracoidal neosternum' of the Amniota. 

 This latter term was based upon his idea that the " interclavicle 

 may be, throughout, the vanishing vestige of the coracoidal 

 sternum of the Ichthyopsida." The acceptance by Parker 

 ('91) and by Howes ('91) of this division of two morphologically 

 different sterna in the group of the vertebrates indicates how 

 completely Ruge's theory dominated their thoughts, and the 

 thought and teaching of that day concerning the origin of the 

 sternum. If, however, the conclusions of later workers regarding 

 Ruge's theory prove valid, and all the facts at the present time 

 seem to substantiate their validity as we shall later see, it is no 

 longer necessary to divide the sterna of the various classes of 

 vertebrates into coracoidal and costal, for no sternum is costal in 

 origin, the union of ribs and sternum being but a late and sec- 

 ondary stage in development. This is obviously an important 

 item, if proved, for it enables us to homologize all vertebrate 

 sterna. Heretofore it has been impossible to homologize the 

 sterna of the Ichthyopsida and the Amniotes because of their 

 supposed dual origin. One of the objects of the present paper 

 is to determine this matter of a single or dual origin for the 

 sternum and the solution of its homology throughout the 

 vertebrates. 



